"Trailer-hitching" bucking problem, Rapid detrioration from running perfectly Options

[boxster914]

Start with fuel filter? Sounds like a progressively clogging fuel issue to me

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Start with some basic maint items. If the 914 sits for long periods of time, not unreasonable to think crud could develop in the fuel tank.

I typically drive the car multiple times a week. It rarely sits undriven for more than a week.

[wonkipop]

thanks for putting up that video.

its hunting as its trying to idle.
i'd go looking carefully over fuel first.
check the filter.
check the fuel pressure if you have a guage and can do it easily.

i've got a similar thing mine does when it gets warmer here.
and its sounds just like that.
i think in part is due to having a rhd car. took me a long time to work out what it was.
without going into detail too much what happens is my fuel overheats due to the very small volume of fuel in the circuit (the bulb at bottom of tank is way smaller in a rhd car).
combined with a new fuel pump in original position it can cause bubbling in the lines etc and fuel starvation. fill the tank up to halfway and its gone just like that. only in hot weather. but its exactly that kind of hunting.
so i would go there first. not saying your problem is that, but not getting adequate fuel through will do something very similar.

an air leak in a L jet will also give you that but tends to go up and down in revs a bit more than that. not really in the know about d jets and air leaks etc.

but that sure sounds like fuel issue from your vid.

it could be that its got clogged injectors, but the only way to know for sure is to pull them and get them tested. maybe leave that until a bit further along.

good luck with it.

stay methodical.

all other suggestions above are correct.
just go through everything one thing at a time.

Hi wonkipop. My '74 2.0 with FI is having the same symptoms boxster914 and you have described. You're reminding me that I've observed very small bubbles in my fuel line. Here's what happened: I replaced the fuel filter which of course drained the line to the engine, so I wanted to purge the fuel line of trapped air. I separated the fuel line at the cold start valve and attached a clear tygon line which I put it into a clear jar, then bumped the fuel pump (by turning ignition on and off). I could see bubbles traveling through the tygon tubing, which I expected. Thing is, I repeatedly turned the fuel pump on and off and I continued to get very fine bubbles in the line.

I'm in California where the ambient temp is around 75. The car has a 2 port fuel pump installed below the fuel tank. The missing happens when the car is warming up so I don't think this is a fuel heating up issue. I'm wondering if I have some sort of leak in the fuel line, something that's letting air in but no fuel out as far as I can tell?

By the way, the tank is near full and I verified fuel pressure is correct and does not fluctuate when the missing occurs.

different problem. not sure how air bubbles would be forming in yours at ambient.
after the fuel pump? the original three port fuel pumps actually use the little sliding valve thats in the head of the pump to release air bubbles and send them off down the return line to the fuel tank (the return line being the line used to blow excess pressure out of the fuel pump). you need a diagram of the port assembly to understand it.
more modern in line type pumps should be releasing air bubbles back through the pressure side port into the fuel pump. if there is bubbles in the line after the pump the pump is not good. bubbles can be in the line before the pump and can be there for all sorts of reasons. heat etc. but should not be there after pressurization.

mine is related to actually running out in traffic. sitting at the lights, where the car, since its virtually original in all respects regarding engine etc, idles hotter. the L jets were/are known for idling hot due to retarded ignition for emissions at idle. so if its a hot day here, and by that i mean 90F + (in imperial) i think the fuel starts to boil in the line before the pump which is under the engine in original spot. (i have steel lines now which i think exaggerate the issue compared to original plastic lines). the reason it can boil is if the fuel volume gets smaller - enough of the fuel recirculating does not gain from a sufficient heat sink in the fuel tank itself. the bulb like area at the bottom of the fuel tank is reduced to half the size it is in a lhd car. soon as i increase the fuel volume by taking the tank to half full the problem goes away as the fuel volume decreases the fuel temp enough to keep it under the threshold in the line before the pump. it happens in my car if i get down to 1/4 of a tank on hot days. so mine is not so much bubbles after the pump but pressure drops due to interrupted supply flows.

i'm not saying this is boxter's problem. its just where i would start first with his car if i was him. just making sure he is getting fuel ok.

if boxster wants to solve his problem r_bowie's advice is very good.
go to the thread.
the other thing boxster needs to do is down load the factory work shop manual.
its got all the trouble shooting pages etc in it to methodically go through.
as soon as Van got hold of that manual he was off and running.
i think its the guy who has the 914/6 GT website. he has the manual there i believe.
i'd have to check again. you need a good manual if you are sitting at home trying to solve these things.

EDIT.
i don't have the problem of the car dying when i give it to it.
which is boxter's problem.
his was able to still idle but as soon as he gassed it, it died.
and now it won't go.
so i would be starting with fuel first and tick that off the list.
could be his fuel pump died.
usually they just go bang.
but a guy i know with a big citroen had his fuel pump go gradually.
and it did more or less what boxster is describing.
after he pulled it out, he tested it and it still ran but it just wasn't building pressure.
something had let go inside it in the pump section itself rather than the motor.
unlike me he never did the can opener trick to find out what it was.
just replaced his fuel pump and moved on.
with the original three port style fuel pumps that 914s had they just go bang.
cease operations. usually due to chewing up the brushes or something in the motor finally after 1 million miles. but a more modern fuel pump can fail gradually or give more warning its on the way out.

[wonkipop]

@boxster914

not sure if you have factory manuals - if you don't and want them go here.

larry lee has them on his website. useful at times like this.

http://p914-6info.net/Manuals.htm

[Lockwodo]

Thanks Boxster914, very helpful to have the manuals, it's an amazing resource and insomnia cure (IMG:style_emoticons/default/biggrin.gif)

Wonkipop: I've done some research and it seems there are two ways that air bubbles can get into the gas line. One is a leak in the suction side of the fuel system, i.e. in the fuel line before the fuel pump or in the fuel tank itself, such that air is getting pulled into the line by suction. The other way is a failed injector combustion seal that allows combustion gases to force their way into the injector nozzle.

[wonkipop]

Thanks Boxster914, very helpful to have the manuals, it's an amazing resource and insomnia cure (IMG:style_emoticons/default/biggrin.gif)

Wonkipop: I've done some research and it seems there are two ways that air bubbles can get into the gas line. One is a leak in the suction side of the fuel system, i.e. in the fuel line before the fuel pump or in the fuel tank itself, such that air is getting pulled into the line by suction. The other way is a failed injector combustion seal that allows combustion gases to force their way into the injector nozzle.

you are thinking of diesels. or direct injection. injectors in a 914 are in the intake outside the combustion chamber.

lets not digress from the man's problems.
he needs to check he is getting fuel through and its at pressure and tick that off the list.

[BillJ]

This exact behavior happened twice in my 75. Went through two filters before simply changing out the tank. Never had another issue.
Changing just the filter may solve though.

[rhodyguy]

Could the tank well for the supply be filled with crud/rust? Have you drained the tank and looked inside of it with a flashlight? What is the condition strainer sock in the bottom of the tank? Kind of sucked flat?

[Root_Werks]

This exact behavior happened twice in my 75. Went through two filters before simply changing out the tank. Never had another issue.
Changing just the filter may solve though.

Good point, don't forget there is a fuel strainer inside the tank that probably hasn't seen the light of day in almost 50 years.

[Jamie]

This exact behavior happened twice in my 75. Went through two filters before simply changing out the tank. Never had another issue.
Changing just the filter may solve though.

Good point, don't forget there is a fuel strainer inside the tank that probably hasn't seen the light of day in almost 50 years.

When I moved the pump to the front I eliminated the troublesome sock filter in the tank, then a fuel filter before and after the pump. This allows easy access and inspection of any line junk that might come from the tank, which I had cleaned. (IMG:style_emoticons/default/aktion035.gif)

[Lockwodo]

This exact behavior happened twice in my 75. Went through two filters before simply changing out the tank. Never had another issue.
Changing just the filter may solve though.

So I monitored the fuel pressure when the engine is running and the missing issue is happening. It's rock steady at 29 PSI. So doesn't that rule out the problem being related to a clogged fuel filter (which I replaced anyway) or crud in the fuel line/tank?

[DRPHIL914]

lots of good advice here already, if the fuel pressure is constant its probably not a blocked line or filter.
the hunting issue makes me think its running lean, but the missing makes me think more on the lines of intermittent short, like bad grounds or loose FI grounds, i had this happen 2x and now my ground wires for the FI are fastened securely to the spade connector thats on the back of the engine case bolt. check that. also - if its on acceleration the TPS could be an issue so you can unplug that to take it out of the equation, also take the cover off of it to chec the TPS board for wear, corrosion etc.
next item is the MPS- ihve had several fail we can go over that later. Go back to the distributor , there is a trigger points plate that fires the injector pulses and those need to be checked and cleaned , and inside your distributor the ground strap could be worn and shorting as well, so check these items and get back to us.
you can test each of these items so get your multi meter out. and vac test the MPS>


Phil -

[BillJ]

Sounds like fuel delivery to the rail is fine. Good to eliminate that!

[emerygt350]

lots of good advice here already, if the fuel pressure is constant its probably not a blocked line or filter.
the hunting issue makes me think its running lean, but the missing makes me think more on the lines of intermittent short, like bad grounds or loose FI grounds, i had this happen 2x and now my ground wires for the FI are fastened securely to the spade connector thats on the back of the engine case bolt. check that. also - if its on acceleration the TPS could be an issue so you can unplug that to take it out of the equation, also take the cover off of it to chec the TPS board for wear, corrosion etc.
next item is the MPS- ihve had several fail we can go over that later. Go back to the distributor , there is a trigger points plate that fires the injector pulses and those need to be checked and cleaned , and inside your distributor the ground strap could be worn and shorting as well, so check these items and get back to us.
you can test each of these items so get your multi meter out. and vac test the MPS>


Phil -

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A quick thing to try that might tell us a little is have a buddy with some starter fluid or just a gas spritzer and see if you can rev it with that (or just rev it by hand in the engine bay while adding go juice). If it revs happy that points to injector issues (trigger, tps, mps, injectors), if not, ignition type stuff might be the issue (less likely).

[wonkipop]

ok

fuels good.

ignition system next.

check the points first. are they clean.

then as above suggestions. go through the rest of the distributor and d jet stuff with the d jet guys.

[Lockwodo]

Yes indeed.

Note to boxster914: I hope my car issues and the discussion in this thread are additive and helpful to you in resolving the issues your car is having. You started this thread and I didn't mean to hijack it (IMG:style_emoticons/default/sawzall-smiley.gif)

Note to everybody: you guys are great and your help/experience is much appreciated! I've checked and tested many of the things recommended and am working through the rest.

[boxster914]

Thank you all for your input (and @lockwodo I appreciated your contribution).

I am compiling a list of things to check which is exactly what I needed.

I will start work this weekend and be back in touch with my findings.

There may be some newb questions in the process. . .

[Dave_Darling]

On a D-Jet car, the hesitation that happens when the throttle position switch is worn has been described as "trailer-hitching". This typically happens at steady-state cruise with a constant throttle position.

Unplug the throttle switch to see if the problem goes away. Don't worry that idle is a little funky, and that acceleration is slower than usual. Just check steady-state to see if the problem is there. If it is, then the throttle switch isn't the problem.

--DD

[Lockwodo]

lots of good advice here already, if the fuel pressure is constant its probably not a blocked line or filter.
the hunting issue makes me think its running lean, but the missing makes me think more on the lines of intermittent short, like bad grounds or loose FI grounds, i had this happen 2x and now my ground wires for the FI are fastened securely to the spade connector thats on the back of the engine case bolt. check that. also - if its on acceleration the TPS could be an issue so you can unplug that to take it out of the equation, also take the cover off of it to chec the TPS board for wear, corrosion etc.
next item is the MPS- ihve had several fail we can go over that later. Go back to the distributor , there is a trigger points plate that fires the injector pulses and those need to be checked and cleaned , and inside your distributor the ground strap could be worn and shorting as well, so check these items and get back to us.
you can test each of these items so get your multi meter out. and vac test the MPS>


Phil -

I recently replaced the TPS board and calibrated it. Disconnecting the electrical connector to the TPS doesn't affect the missing issue at all, so the missing isn't due to the TPS.

I did some tests on the MPS today and here is what I found: 95 ohms resistance between terminals 7 and 15, 340 ohms resistance between terminals 8 and 10, and no continuity between the coils and the case of the MPS. All good so far. Then I hooked a vacuum pump to the MPS sensing port and pulled 10 inches of vacuum. After one minute, the vacuum had dropped to 7 inches of mercury, then to 6 inches after a total of 2 minutes, then to 5 inches after a total of 9 minutes, where it seems to level off.

So the resistance numbers are in spec, but is the vacuum loss over time acceptable? I understand that even if the vacuum loss is acceptable, the MPS can still have a defective aneroid cell. What do you think? Perhaps not worth testing it further at this time.

I'm working on the ground points and distributor next.

[boxster914]

OK so I have some results to report on.

I changed the fuel filter.

After the filter change the car had 3 instances of minor bucking (much less than the original problem) early on in a 20 minute drive and then it ran fine for the rest of the time with no bucking.

Although after I got home there was some idling dropouts as captured on this video:

https://youtu.be/u-ttZXnvmOk

Notice the drop out at 0:02 and 0:10.

I figured the problem was not solved so I moved on to thinking it is caused by a vacuum leak.

I consulted the excellent vacuum hose diagram at bowlsby.net

https://bowlsby.net/914/Classic/zTN_FI_Hose....7L_1970-71.jpg

I changed the 4mm line (yellow in the diagram) after the fuel filter change and took the car out for a 10 minite run and there was no bucking at all.

Here is a picture of the new line:

At the throttle body


At the vacuum can


I welcome your feedback.

What is the function of the 4mm vacuum line that I changed? I don't know what it does. Could this have been the problem?

Thank you for all of your help.

[wonkipop]

if its anything like a L jet, its the vac retard line from the distributor to the throttle body.
not sure exactly how they work with D Jet but know in L jet its to retard the ignition at idle mostly. it retards timing when there is engine vac, in L jet its on the manifold plenum side of the throttle body so its functioning when the throttle plate is closed at idle. mostly there to assist with emissions on L jet.

D jet guys would be able to tell you exactly how it works on your car.

but aside from it possibly affecting the retard which i don't think is significant in terms of sitting there idling it would have been an air leak into the system.

combine that with your fuel pressure or fuel restriction problem and your mixture leaned right out. why it wouldn't idle, hard to start and was bucking.

you have probably been driving around the minor air leak (and it does look minor) for a while and not even noticing it. i think your main problem might have been fuel restriction. the air leak would have exaggerated it, perhaps not a lot.

good work solving it.

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EDIT
ps
when i think about things back two years ago the problem i had was very similar.
even though i have L jet.
a combo of fuel and air leak.
i found the air leak first. it was the throttle body gasket had done hard.
as soon as the engine really warmed up thermal expansion difference between the throttle body and the plenum caused the leak. it wasn't much of a leak i thought but it was around probably 1./2 the perimeter of the throttle body. new throttle body gasket and the problem seemed to go away. a bit like yours subsided after the filter change. ....that was until i ran the gas down to a 1/4 of a tank.
see my posts earlier in this thread. i am glad your fuel problem was a little easier to solve. my own fault for "butchering" the car to convert it to rhd 30 years ago. (IMG:style_emoticons/default/smile.gif)